Orthogonal frequency division multiplexing (OFDM) is a basic transmission mode in current wireless communication, and is widely applied to wireless communications systems such as Long Term Evolution (LTE), Worldwide Interoperability for Microwave Access (WiMAX), and Wireless Fidelity (WiFi). In addition, the OFDM is further applied to fixed network transmission, for example, transmission modes such as an optical fiber, a copper twisted wire, and a cable. A basic principle of the OFDM is: when it is ensured that subcarriers are orthogonal, a spacing between the subcarriers is compressed to the smallest. In this way, it can be ensured that multiple parallel paths that do not interfere with each other are formed, and in addition, frequency usage efficiency of a system is improved. Because the OFDM has the foregoing features, if OFDM subcarriers that do not interfere with each other are allocated to multiple users, the OFDM can be used to implement multi-user access or multi-user data transmission. It can be learned that OFDMA may be used to implement parallel multi-user data transmission, and data transmission concurrency is improved.
In addition, a multiple-input multiple-output (MIMO) technology can provide transmit (receive) beamforming, so as to effectively improve transmit (receive) power, and effectively improve reliability of a communications system. Furthermore, the MIMO technology can generate additional spatial freedom, so as to improve a throughput of the system many fold, and effectively improve a rate of the communications system. Because of these advantages of the MIMO technology, the MIMO technology has become one of critical technologies in the 802.11n standard protocol and the 802.11ac standard protocol. In addition, because a beamforming technology is used, a transmit end may send data to multiple users by using multiple spatial flows, or may receive data sent by multiple users in different spatial flows, so as to implement parallel multi-user data transmission and improve data transmission concurrency.
Currently, an orthogonal frequency division multiple access (OFDMA) mode, a multi-user MIMO (MU-MIMO) mode, or an OFDMA and MU-MIMO hybrid transmission mode is commonly used in uplink multi-user transmission. A station (STA) needs to learn of a transmission configuration and a parameter of the station by using an access point (AP), for example, a specific used spectrum resource block, a quantity of used spatial flows, a used modulation and coding scheme, and time synchronization information. Therefore, a manner in which the access point triggers the station to perform uplink multi-user transmission draws extensive attention. However, for uplink multi-user transmission triggered by the access point, if the station does not know a time at which the access point sends a trigger frame, the station needs to keep listening to a channel so as to receive the trigger frame to perform uplink transmission. This is inconvenient for power saving of the station.
In the prior art, an access point may periodically broadcast a trigger frame scheduling information element by using a beacon frame, and the trigger frame scheduling information element carries a sending time of a first trigger frame and a quantity of trigger frames that are to be sent after this beacon frame. Further, when sending a trigger frame, the access point may add a sending interval between a sending time of the trigger frame and a sending time of a next trigger frame to the trigger frame, that is, after the sending interval, the access point sends a trigger frame again.
However, if the access point adds a sending interval between a sending time of the trigger frame and a sending time of a next trigger frame to each trigger frame, because triggering signaling of the trigger frame is in a physical layer preamble of the trigger frame, and this part of signaling resources are relatively precious, relatively large signaling overheads are caused. If the station does not successfully receive a trigger frame, the station cannot obtain a sending time of a next trigger frame. Consequently, reliability is relatively poor. Therefore, the present disclosure is to achieve a technical effect that when signaling overheads are not increased and reliability is ensured, a station learns of a sending time of a trigger frame, and the station remains in a receiving state in an appropriate period of time.